(A) The amino
end on the second (R2)
amino acid is added to the carboxyl endof the first (R1)
amino acid.
In the in vitro reaction, an O from
the carboxyl end combines with two protons (H)
from the amino end to form a single water molecule (H2O).
The dehydration reaction leaves a covalent peptide
bond (O=C-N-H).

(B) Because successive amino acids are added to
the carboxyl end of the polypeptide, and the
amino terminus of the R1
amino acid remains
unchanged, the polypeptide is said to grow in the N
C direction.
Each of the repeating ( -N-C(R)-C- ) subunits
remaining after the dehydration reaction is an amino
acid residue.

Note: The diagram above shows the formation of
a peptide bond in vitro by a dehydration reaction
that splits out an H20
molecule. The same bond is formed in vivo by a
condensation reaction,
in which no molecules are lost.